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Know what makes this Friday special? It’s not that it’s the start to a glorious three-day weekend (although that certainly helps). Rather, Friday marks the last blue moon you can expect to see until 2015.

Now, we don’t want to mislead you. “Blue moons” aren’t so-named because the moon turns azure or gives off a blue-y radiant hue. Instead, the term “blue moon” refers to the fourth full moon in a season or the second full moon in a month. We had a full moon Aug. 1, making tonight’s mooning a blue one.

The last blue moon was on New Year’s Eve on Dec. 31, 2009, and the next one isn’t projected to happen until July 31, 2015. So if you buy into all that Mayan apocalypse stuff, this may well be your last chance.

Blue moons occur because our calendar months don’t sync exactly with the orbit of the moon (it takes the moon 29.5 days to move from full to new to full again, and calendar months are usually longer than that because they are based on the solar cycle), although many astronomers contend that the modern definition came about due to an old error in the Maine Farmer’s Almanac. Lucky for us, the error made the blue moon a far more frequent thing to behold.

So whether you’re more interested in a neon moon or the actual moon, we hope you take some time to duck outside tonight and appreciate this astronomical rarity. The moon will be at its fullest at 8:58 p.m. Central Standard Time.

Jupiter dominates this month’s evening skies. It outshines all stars in the sky, so it’s easy to find. Face south at dusk and look for the brightest thing there.

Mercury has emerged into the evening sky, and is visible at the beginning of this month. Look low in the southwest at dusk, right over the point of sunset. By mid-month, Mercury is again lost in the Sun’s glare; it re-aligns with the sun (is at inferior conjunction) on Dec. 19.

Mars is now lost in the sun’s glare; it will remain invisible to us all winter as Earth passes around the far side of the sun from it.

Look for the enormous Summer Triangle in the night sky, consisting of the stars Deneb, Vega, and Altair, in the west. These stars were up all night long back in June and July, hence the name. The Great Square of Pegasus, not quite as bright as the Summer Triangle, is high in the south at dusk. The star in its upper left hand corner is also the head of Andromeda. Rising after Andromeda is Perseus, the hero that saved her life. Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen. Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from that Dipper. Taurus, the Bull rises in the northeast. Look for the Pleiades star cluster at the feet of Perseus. Dazzling Orion, the Hunter rises shortly after dusk (by month’s end, it is already up at dusk). As Orion enters the evening sky, we transition from the relatively dim evening skies of autumn to the brilliant stars of winter.

The full moon of early Tuesday, December 21, enters the Earth’s shadow, causing a total lunar eclipse. This eclipse is visible in its entirety from all of North America, including Houston. The moon first encounters the Earth’s shadow (umbra) at 12:32 a.m. This marks the beginning of the partial eclipse. The moon takes just over an hour, until 1:40 a.m., to enter the shadow. That is when totality begins. In this eclipse, the Moon does not quite cross the center of Earth’s shadow but instead passes through the northern part of it. Even so, the moon takes 74 minutes to cross to the other side of the shadow, so totality lasts from 1:40 to 2:54 a.m. By 4:02 a.m., the moon has re-emerged from the shadow, and the eclipse is over. Remember, seeing a lunar eclipse requires no special equipment at all; anyone who sees the moon sees the eclipse. The only thing that could stop us from seeing this would be a cloudy night on December 20-21, 2010. The next total lunar eclipse we see here in Houston occurs just after midnight on April 15, 2014.

At 5:42 p.m. on Tuesday, December 21, the sun is overhead at the Tropic of Capricorn, the most southerly latitude where the sun can be overhead. This is therefore the winter solstice for those of us in the Northern Hemisphere, and the summer solstice for people south of the equator.

At Houston’s latitude, the earliest sunset of the year occurs Thursday, December 2. Of course, days continue to shorten until the solstice, which makes sunset earlier and sunrise later. However, Earth is also accelerating as it approaches perihelion (closest approach to the sun) in early January. This causes sunrise, local noon, and sunset to occur slightly later each day. This close to the solstice, the second effect actually predominates, so sunset gets a little later during December even while the days are getting shorter. As you head out to ring in the New Year, notice that sunset on New Year’s Eve is about 10 minutes later than it is now.

The Geminid meteor shower peaks every year in mid-December, this year on the 14. This shower and the Perseids in August are the two most reliable showers of the year, producing about 1 or two meteors per minute. The Geminids are not as popular, though, because of colder nights (yes, sometimes even in Houston) and a greater chance of cloudy skies. Still, it’s worth a look if the skies are clear. Unlike most meteor showers which are comet debris, the Geminids originate from an asteroid (3200 Phaethon. The shallower angle between this debris path and Earth’s orbit means that Earth rotates us towards the debris field before midnight. We can thus observe meteors from late evening all the way until dawn. Meteors will seem to radiate from the constellation Gemini, hence the shower’s name.

This month the great planet race continues, as Venus, Mars and Saturn form a triangle in the west. Watch the triangle change shape each night as Venus overtakes Saturn and then Mars!

Venus is by far the brightest of the three planets. Face west at dusk and look for a point of light that outshines everything in the night sky.

Saturn and Mars are to the upper left of Venus as August opens. Mars is below Saturn and a bit to its left. Although these two planets of similar brightness are much dimmer than Venus, they outshine all the other stars near them.

Observe all three carefully throughout August and watch as their configuration changes. Mars aligned with Saturn last Saturday (July 31) and now begins to move farther to Saturn’s left. Venus, moving faster than the other two, continues to approach from the right; it will pass Saturn on August 8. Venus then continues to gain on Mars as they both move away from Saturn. Venus finally overtakes Mars on August 19-20. On the night of August 31, Venus and Mars are to either side of the star Spica in Virgo.

Jupiter is now a late evening object, rising by 11 p.m now and by 8:45 p.m. at month’s end. It outshines all stars in the sky, so it’s easy to find. Face east in late evening or south southwest at dawn to see it.

The Big Dipper is in the northwest at dusk. You can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica’. These stars are in the west at dusk tonight. Arcturus, by the way, is the fourth brightest star we ever see at night, but the brightest one Americans ever see on an August evening. Spica is in Virgo, the constellation where this month’s ‘planet race’ occurs.

In the east, look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair. This triangle is up all night long from June to early August, hence its name. Scorpius, the Scorpion, is in the south at dusk. Sagittarius, the Archer, known for its ‘teapot’ asterism, is to its left. Between these two star patterns is the center of our Milky Way—the brightest part of that band as wee see it. On a cloudless night far from the big city, see if you notice the Milky Way glow near the ‘teapot’ of Sagittarius. In late evening, look for the Great Square of Pegasus rising in the east.

On Friday morning, August 13, the Earth passes through a stream of debris left long ago by Comet Swift-Tuttle. This produces the Perseid Meteor Shower, one of the best meteor showers each year. The Perseids occur every year at about this time, producing on average about one meteor per minute. Keep in mind that even a short period such as a minute can seem longer if you are waiting for something to happen. Since Earth is running into the meteors, not the other way around, the leading edge of the Earth encounters the shower. This is the side going from night into day. Accordingly, we see more meteors as dawn approaches. Big city lights or the Moon can limit the meteors you see by dimming out fainter ones. This August, however, the New Moon is on the 10th, giving us a skinny crescent on the 12th which sets long before the shower really gets going. The main challenge, then, is to avoid city lights.

If skies that night are clear, our George Observatory will open Thursday night, August 12 at 9pm and remain open until dawn for observing the shower. If you come out to George or go elsewhere, you’ll want to lie on your back (to see as much of the sky as at once as possible) and orient yourself towards the constellation Perseus. (The shower is called the ‘Perseids’ because they seem to radiate from that constellation.) Perseus rises in the northeast at dusk and is high in the north at dawn.

The full moon of Jan. 15 partially blocks the sun, causing an eclipse visible in Africa and Asia. Because it is close to apogee, the moon is too small in the sky to ever block the sun completely, and no total eclipse occurs. Instead, folks on a path stretching from Uganda and Kenya across the Indian Ocean to Sri Lanka, Burma, and China see an annular eclipse. The moon will appear to be completely inside the sun’s disk with a ring of sun around it.

At about 6 p.m. on Saturday, Jan. 2, the Earth came to perihelion. This means it was as close to the sun as possible—about 147 million km away (Earth is about 152 million km from the sun in July). This is not enough of a difference to influence our weather. Earth’s 23.5 degree tilt is much more important. Our North Pole is still tilted very much away from the sun and the sun still takes a very low path across our sky. Therefore, in spite of being as close to the sun as we’ll be all year, we’re going to be quite cold in Houston this week.

The latest sunrise of the year (at Houston’s latitude) occurs on Jan. 10. Earth is now just past perihelion, and has sped up a little in its orbit since it is a little closer to the sun. As a result, sunrise, local noon, and sunset have been happening a little later each day since early December. The noon sun is ever so slightly higher at noon each day since the solstice on Dec. 21, but as of today, the sun is still very close to its solstice height. Beginning Jan. 11, the noon sun’s greater height in the sky again becomes the dominant effect (as it is for most of the year). From then until June, sunsets are getting later while sunrises are getting earlier.

Jupiter is the brightest thing in the evening sky this month, except for the moon. You can easily see it by looking to the southwest. Jupiter appears lower and lower to the horizon each January night. By the end of the month, Jupiter sets just after 7 p.m. At the end of February, Earth and Jupiter will be on opposite sides of the sun and Jupiter will therefore be invisible to us.

Mars has become an evening object. It now rises in the east about 8 p.m. and will rise by dusk at the end of the month. Mars already outshines all stars in the night sky except the very brightest (Sirius), and will continue to brighten throughout January. On Jan. 29, Earth will pass between Mars and the sun, putting Mars in our sky all night long (this alignment is called opposition).

The Great Square of Pegasus is in the west at dusk. The star in its upper left hand corner is also the head of Andromeda. Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen. Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from that Dipper. Since the Dipper is low and out of sight at dusk this month, Cassiopeia rides high.

Dazzling Orion is high in the southeast, reminding us that winter is here. His belt points up to Aldebaran, the eye of Taurus, the Bull. The Dog Stars Sirius and Procyon are below Orion in the east. Sirius is the brightest star we ever see at night. Gemini, the Twins, are to Orion’s left as he rises (and to his upper left once they appear to the south). Look for two stars of equal brightness less than 5 degrees (three fingers at arms’ length) apart. These are Castor and Pollux, marking the twins’ heads. High in the northeast is Capella, the sixth brightest star ever seen at night.

Moon Phases in January 2010:

Last quarter moon January 7, 4:41 am
New moon January 15, 1:12 am
First quarter moon January 23, 4:53 am
Full moon January 30, 12:18 am